We researched the factors affecting energy consumption in a real apartment house with a gas hot water floor heating system. Our research investigation was conducted on the basis of a questionnaire including questions pertaining to lifestyle, the usage of heating equipment, and the measurement of energy consumption on the basis of gas and electric power. We also calculated the heating load by computerized simulation in 40 different cases. We found that the heating consumption of an edge side unit group, with many walls touching the air, exceeded that of a hot water supply. In addition, the heating load was varied in each condition of the computerized simulation. For energy saving, we must plan the best heating system and the best insulating design at each unit location.